Not supported. We don’t know Garber’s opinion about the frequency of speculative bubbles. We only know what he thinks about whether the Dutch tulip market was a speculative bubble.

Supported. Garber offers the potential for earnings from selling copies of the original bulb as a reason that the initial purchase at a high price does not have to be irrational. This suggests he thinks people who bought the original bulb could have rationally expected not to lose money as long as they sold copies made from their original bulb.

Not supported as something Garber would agree with. Garber thinks that if the prices are not irrational, then there’s no speculative bubble. But (C) reverses this relationship. Garber doesn’t have to think that if there’s no speculative bubble, the prices are not irrational. Maybe he could think prices can be irrational for other reasons unrelated to the presence or absence of a speculative bubble.

Not supported as something Garber would agree with. We don’t get any evidence of Garber’s thoughts concerning “all” the investments of those who invested in Dutch tulip bulbs. Although we know what he thinks about their investment in Dutch tulip bulbs, this doesn’t constitute evidence of what he thinks about other kinds of investments.

Not supported. Garber thinks Mackay is wrong to believe that the original high prices were irrational. But there’s no evidence Garber thinks Mackay believed the later, low prices were irrational.